Lead bromide crystals are used to produce an acousto-optical tunable filter of very high optical quality. These crystals are used to filter laser light waves that pass therethrough without distortion. Any non-homogeneity severely diminishes the effectiveness of such a crystal.
In a crystal growth furnace, materials are placed in an ampoule heated until liquified and cooled under controlled conditions allowing a crystal to grow. Careful control of the heating and cooling conditions improves the quality of the crystal. The crystals begin to form at a solidification point during the growth process. At this point an interface exists between the liquid material and the crystallized material. The shape of the interface is directly related to the quality of the crystal formed. The most preferable shape is a flat surface (flat interface) between the liquid and solid phases. A flat interface indicates a crystal free of impurities and defects in its structure. Convex and concave shapes indicate the likelihood of thermal stresses which can decrease the quality of crystal.
Traditionally, a two zone Bridgman type furnace is used for lead bromide crystal growth. To achieve requisite crystal homogenity, for the acousto-optical tunable filter application, the crystal growth must exhibit a flat interface between the solid and liquid phases. The Bridgman furnace is a vertical furnace with a hot and cold zone. The hot and cold zones create a thermal gradient therebetween. Insulation is used in traditional designs in and around the furnace maintaining axial heat flow to create a nearly-flat interface. The presence of insulation prevents observation of the interface and adjustment of the thermal characteristics of the furnace to compensate for any change in the interface shape. Thus, the traditional Bridgman furnace lacks the versatility for optimal control of crystal growth parameters necessary for producing crystals of sufficient quality for acousto-optical tunable filters.
Visual monitoring of the interface is important for commercial applications and as a research tool to study interface dynamics. Visual monitoring allows the observer to adjust the thermal profile inside the furnace in order to insure high quality crystals. Also, for educational purposes, observation of the crystal as it is formed allows students to observe the relation between the thermal conditions within the furnace and the interface shape.
Multiple-zone furnaces can be used to create individual heat zones capable of controlling the growth of the crystal without using insulation between the zones. It is known to use proportional, integral, derivative controllers to regulate the temperature at each zone. Proportional, integral, derivative control cannot account for zone to zone thermal interaction and the thermal inertia of the ampoule as it translates through the furnace. Traditionally, multiple zone furnaces have failed to provide a controller that accounts for these thermal interactions and thus, the parameters to grow crystals were severely limited. Thus it is desirable to have a system that will precisely control the zone temperatures accounting for zone to zone thermal interactions and the thermal inertia of the ampoule.
Crystal growth is generally performed by moving an ampoule containing the crystal growth material physically through the hot and cold zones. Crystal quality and interface shape can be affected by any disruption of the movement of the ampoule through the heating zones. The crystal growth process requires the ampoule move at an extremely slow pace through the furnace. At times, the growth process can take up to a week or more. It is known to use stepper motors to control the translation of the ampoule. The stepper motor allows very slow translation through the heat zones, but produces a jerky motion of the ampoule. The jerks created by a stepper motor are detrimental to the quality of the crystal produced for extremely slow translation rates. Thus it is desirable to use a translation system that provides slow continuous (jerk free) motion.
The rate of translation through the zones can affect the growth rate and interface quality of the crystal. To control the growth rate and interface quality, it is desirable to control translation in response to changes in the thermal dynamics within the furnace.
To avoid moving the ampoule through the heat zones physically, it is possible to create an electro dynamic gradient. In effect, the electro dynamic gradient moves the thermal profile past the ampoule without having to move the ampoule physically. Multiple zone furnaces are capable of this effect. However, moving the heat zones increases the zone to zone heat interaction in the furnace. Traditional furnaces do not account for these interactions and the crystal quality suffers. It is desirable to create a crystal growth system accounting for the zone to zone thermal interactions to produce high quality crystals by means of electro dynamic gradient.
Thus, it is desirable to construct a crystal growth furnace that provides for visual inspection and precise control of the thermal gradient accounting for zone to zone thermal interactions and thermal inertia created by the ampoule.